Compact polymer lens

ABSTRACT

The invention relates to an optical element having adjustable focal length comprising a first transparent layer having a chosen flexibility and being provided with a piezoelectric layer adapted to contract upon the application of an electrical voltage and thus bend the first transparent layer, the piezoelectric layer being symmetrically positioned relative to an axis, and wherein the first transparent layer is applied on a substrate having a through going cavity positioned essentially symmetrical relative to said axis, and a transparent polymer being positioned in said cavity having a surface contact with said first transparent layer.

The present invention relates to a lens with adjustable focal length comprising a first transparent layer and a transparent, flexible material of a soft polymer or similar having a chosen refractive index positioned on a substrate, and methods for producing the lens.

With the recent developments in optical equipment, such as cameras in mobile phones, scanning apparatus and machine vision, there is a demand for small lenses being capable of fast focusing. In mobile phone cameras the number of pixels has been increasing but there is a demand for compact lenses of sufficient quality to use the full advantage of the pixels. This requires focusing capabilities in addition to small size, especially if the camera is also adapted to other purposes, such as reading bar codes and scanning images of objects close to the camera. Adding focusing capabilities to the lens also allows for use of larger apertures, thus increasing the light sensitivity of the system without suffering from the reduced depth of field of the lens.

Conventional glass lenses have been regarded as to large and expensive for many purposes and research has been may to find other solutions. One promising area has been in the development of lenses made from soft polymers. These have some optical properties and may be shaped for focusing action by electrostatic forces, stretching the soft polymer lens or by shaping the soft polymer surface to obtain a chosen shape. Another proposed solution has been to use a soft polymer with a graded refractive index, but this has turned out to be complicated to produce in sufficiently good quality. The problems related to these solutions have been to obtain a sufficiently good lens surface, both in the curvature and the surface continuity.

Other proposed solutions incorporate using a liquid placed in a lens like cavity where the shape of the cavity is adjusted to adjust the focal length of the lens. Examples showing this are discussed in Japanese patent applications, publication Nos JP2002239769, JP2001257932, JP2000081503, JP2000081504 JP10144975 JP11133210, JP10269599 and JP2002243918. In addition this is discussed in a paper by T. Kaneko et al: “Quick Response Dynamic Focusing Lens using Multi-Layered Piezoelectric Bimorph Acutator”, Micro-Opto-Mechanicle Systems, Richard R. A. Syms Editor, Proceedings of SPIE, Vol. 4075 (2000). All of these are based on a liquid confined inside a cavity which acts as a lens and where at least one of the surfaces may be shaped by an applied force. This has the disadvantage that the pressure applied to shape the lens has to compress the fluid or the cavity, which requires large forces, or additional chambers have to be provided so as to contain the liquid forces out of the cavity. Volume changes due to temperature fluctuations may also cause problems.

Thus it is an object of this invention to provide a compact focusing lens which may be mass produced and compact equipment such as mobile phones, while providing sufficient optical quality with a large range of distances from the camera. This is obtained as stated in the independent claims.

Note that the term “soft polymer” in this specification is used in a wide sense of the word, and may include a number of different materials, such as silicon and polymer gels.

Using a soft polymer makes it possible to produce lenses where the polymer is in contact with air or other compressible gases, thus requiring much less force when adjusting the focal length of the lens. It also eases the production as the polymer will keep in place even if the different production steps are localized in different positions or facilities. As mentioned above it also makes it possible to provide leakage channels or bubbles of compressible gas in order to reduce the required force necessary to adjust the lens and to reduce the strains caused by temperature and pressure fluctuations in the environment.

The present invention thus provides an advantageous solution by utilizing the characteristics of a thin flexible membrane and the soft polymers.

The present invention especially provides a compact lens with adjustable focal length which is easy to produce in large quantities, being suitable for compact cameras like web cameras, mobile phones or analysing equipment of different types, as the lenses are based on well known technology incorporating few independent parts. For example the lenses may be made by screen printing the piezoelectric layer on the transparent layer and fitting the transparent layer on a silicon substrate with etched openings wherein the soft polymer is positioned.

The invention will be described more in detail below with reference to the accompanying drawings, illustrating the invention by way of examples, wherein

FIGS. 1 a-8 c illustrates different embodiments of the invention.

As is clear from the drawings, the lens according to the invention may be made from a first cover glass 1 made from a flexible material, e.g. SiO₂ or Pyrex, with an optical transparent soft polymer 3 with a known refractive index, e.g. a polymer gel, positioned on the substrate 4 or in a hole in a substrate 2. According to a preferred embodiment of the invention the cover glass 1 is a thin glass plate of the type used in microscopy for holding samples. These glass plates are sufficiently flexible to be formed by an applied pressure, and are sufficiently strong not to break under these conditions and the first optical surface in constituted by the first cover glass, thus representing a formable but fairly hard and durable surface, while the rest of the lens material is constituted by the soft polymer. Preferably the soft polymer and the cover glass have the same refractive index, so at so constitute a single optical surface.

In FIGS. 1 a and 1 b the soft polymer 3 is positioned in a hole in a substrate 2. This embodiment may be produced by using silicon as a substrate. If one side of the substrate is provided with a transparent layer, e.g. SiO₂ or Glass, it may be etched from the opposite side thus producing a recess with an etch stop at the silicon layer. The recess is then filled with a soft polymer 3. The actuator, e.g. a piezoelectric ring, 5 may then be positioned in a per se known way, e.g. by screen printing, on top cover layer 1 and the recess, where the ring is adapted to contract or expand tangentially when a voltage is applied. The thin top cover layer 1 and the actuator work together as a bimorph actuator. In FIG. 1 b the ring 5 is radially contracted producing a bulge in the cover layer 1. Experiments have shown that the SiO₂ layer then provide a curved surface which in the central area constitutes an essential spherical refractive surface 6, thus providing a lens.

In the lower part of FIG. 1 b it is illustrated that the lower surface 7 of the soft polymer is moved upward following the soft polymer being drawn upward into the bulge at the cover layer, thus providing a meniscus lens. In FIGS. 1 a and 1 b the recess has a larger diameter, e.g. resulting from the silicon etching process, and thus the curvature of the lower refracting surface is smaller than the upper refracting surface 6, so that a lens is provided. As mentioned above the experience from other soft polymer lenses with free soft polymer to air interfaces is that the central area of the lower surface will have less curvature than the edges, thus the effect of the curved lower surface 7 is limited. In some situations, if special optical characteristics are required, the shape of the hole could be reversed, having the narrow opening away from the first layer 1.

FIGS. 2 a and 2 b illustrates essentially the same embodiment as in FIGS. 1 a and 1 b, but comprising a second layer 4. In this case the lower lens surface 7 is plane, the lens thus providing a planar convex lens. In the embodiment shown in FIGS. 2 a,2 b the recess edges are parallel to the optical axis, and may be produced by other etching methods, if the made in silicon, or by drilling.

If the second layer 4 is sufficiently flexible and the soft polymer is sufficiently incompressible it will to a degree follow the movements of the first surface, as FIGS. 1 a, 1 b provides the extreme example of, the dimensions of the openings may then be chosen to control this effect and the characteristics of the lens. In the drawing a leakage channel 12 is provided for equalizing the pressure inside and in the environment, and to reduce the required power used to change the lens surface. A small bubble of compressible gas also or alternatively be provided outside the optical axis in the cavity.

As is illustrated in FIGS. 3 a,3 b a concave lens may also be provided by reversing the direction of the force applied by the piezoelectric element. As in FIG. 2 a leakage channel 12 is also provided and in addition the amount of polymer is chosen so as to provide a compressible gas 11 to the present in the cavity. The amount of polymer 3 should of course be sufficient to cover the optically active area of the lens, and the gas 11 should preferably cover a symmetrical space so as to ensure symmetric conditions for the force applied by the piezoelectric elements 5.

The solution illustrated in FIGS. 1 a,b, and 2 a,b may be made from ordinary Si production methods. The lower surface is a plane substrate, e.g. from glass, and the flexible layer is made from a Silicon wafer wherein a hole is made up to an upper SiO₂ or glass surface. The soft polymer is positioned in the hole and is thus squeezed between the substrate (in the case of FIGS. 2 a,b), the silicon disk walls and the upper SiO₂ or glass layer. By providing piezoelectric elements or similar on the surface, the curvature of the surface will be modified by the bimorph actuator, and the soft polymer will stick to the surface and follow the movement of the surface.

In all of the solutions discussed above the adjustable refractive surface 6,7 may be made in a reflective material, thus providing an adjustable mirror. In FIG. 2 and onward both the upper and lower layers 1,4 in the drawings may be flexible, so as to provide bi-convex, bi-concave or meniscus lenses.

The optical element according to the invention may thus be coupled to a system for adjusting the focal length of the lens by coupling the actuator providing the force to the lens to standard equipment for providing auto focus to a camera, e.g. in a mobile phone, scanner or bar code reader. The actuator then applies an increasing or decreasing force to the lens so as to adjust the focal length until a signal is given by the system to stop or go backward, e.g. if the required focus has been passed or the adjustment turns out to have the wrong direction.

FIGS. 4 a,4 b, 4 c illustrates the preferred embodiment of the invention but without showing the substrate, wherein the piezoelectric or electrostrictive material, is screen printed on the upper layer 1, or deposited and patterned through a per se known lithographic process. The actuator layer 5 shown in FIGS. 4 a,4 b, 4 c is ring shaped. According to an alternative embodiment using a transparent actuator, e.g. a polymer film or a PZT film, combined with transparent electrodes, e.g. ITO, the actuator may cover the upper layer completely. The upper layer 1 is made from a thin glass sheet, preferably of Pyrex but sapphire or SiO₂ may also be used. The soft polymer like material 3 is preferably made from a conventional gel, but soft polymers or elastomers may also be used, chosen according to the required refractive index. The lower layer 4 is in this case made from glass. The embodiment according to FIGS. 4 a,4 b, 4 c constitutes a solution being easy to produce using standard processes.

In FIGS. 5 a,5 b, 5 c an embodiment is illustrated for producing a cylindrical lens. The advantage is that the force required to bend the upper layer in one dimension is much less the force required to produce a dome shaped surface. In order to provide the same characteristics as a conventional lens two cylindrical lenses may be positioned in a series along the optical axis with perpendicular bend directions.

Power supply and other electronic and/or mechanical device such as driving circuitry, support and housing for the optical element needed for utilizing the invention is not described in detail as they are considered to the obvious to a person skilled in the art, depending on the chosen embodiment and the situation in which it is to be used.

To summarize the invention thus relates to an optical element having adjustable focal length comprising a first transparent layer and a transparent soft polymer, e.g. a gel, having a chosen refractive index situated thereon, said layer being made from a material having a chosen flexibility, e.g. a thin glass layer, the optical element also being provided with an actuator for applying a force upon said flexible layer, said force being essentially planar or circular symmetric relative to said axis thus bending the polymer surface close to the upper layer providing a lens surface and providing a curved refractive surface.

The optical element may also comprise a second transparent layer positioned on the opposite side of the soft polymer and essentially covering said soft polymer so as to provide a second refractive surface 7 close to the second layer with a predetermined shape. Alternatively the second transparent layer may also be flexible and provided with an actuator for shaping the second refractive surface. If one of the layers has an opening the polymer inside it may be squeezed out of or retracted into the polymer chamber the surface may provide a lens shaped refractive surface, but, as stated above, the optical quality of this surface may be limited.

The first transparent layer may be constituted by SiO₂ and the optical element further comprises a Si substrate having an opening, said soft polymer being situated in said opening.

It is also possibly that a lower lower transparent layer also comprise a piezoelectric element so as to control both optical surfaces of the lens.

Alternatively the force actuator may be mounted in a housing and be arranged symmetrically relative a chosen optical axis and adapted to apply a force being parallel to the optical axis. This may be provided using a ring being forced against at least one of the layers 1,4 to bend the layers, the force may be applied mechanically or with magnets, piezoelectric elements or electrical actuators.

According to a different embodiment of the invention an essentially circular bubble is provided between said first and/or second layer and said soft polymer at a chosen optical axis, said soft polymer being electrically conductive, and said first layer being provided with a ring shaped pattern of electrical conductors symmetrically relative to said chosen optical axis, and said actuator being constituted by a voltage source coupled to said electrodes and to said soft polymer, thus providing a force between a chosen electrode on said first layer and said soft polymer shaping the soft polymer surface relative to said electrodes.

According to another embodiment of the invention the second layer is placed on the opposite side of the soft polymer from said first layer, and the soft polymer is electrostrictive. The actuator is constituted by at least one circular electrode provided on each layer and voltage providing means applying a voltage between the electrodes, the soft polymer thus reacting to the voltage and expanding or contracting as a result of said voltage. This way the reactions of the polymer to the electrical field results in the curved refractive surfaces.

As is described above the actuator is adapted to provide a circularly symmetric force relative to the optical axis, thus providing an essentially spherically shaped bend on said first transparent layer close to the centre. It is, however, possible to let the actuator be adapted to provide a planar symmetric force relative to a plane including the optical axis, thus providing a cylindrically shaped bend on said first transparent layer, where the cylindrical bend has an axis perpendicular to the optical axis. By combining two such lenses along an optical axis a result similar to one traditional lens may be achieved, but requiring less force applied by the actuators.

The lens according to the invention may be produced in several ways. The lens illustrated in FIGS. 1 a-2 b may be made using e method comprising the steps of providing a silicon plate having a SiO₂ or glass layer on one side, etching an opening into said silicon plate, and positioning a soft polymer into said opening, the soft polymer thus having contact with the SiO₂ or glass layer, and positioning a force actuator on said SiO₂ or glass layer. A second layer may then be provided on the opposite side of said silicon plate, the soft polymer thus being enclosed in a cavity. The actuator is a piezoelectric or electrostrictive ring said ring being positioned coaxially with the chosen optical axis and being adapted to provide a radial force to the SiO₂ layer thus making a bulge or dome in said first layer and said soft polymer.

Alternatively the lens may be made by the step of depositing the actuator on the glass surface, e.g. by a printing method, thinning the glass by e.g. etching or grinding from the opposite side of said glass surface, and providing the soft polymer on the opposite side of the thin glass plate. This way the glass will constitute both the substrate 2 and the first transparent layer 1. 

1-14. (canceled)
 15. An optical element having adjustable focal length comprising: a first transparent layer having a chosen flexibility; a piezoelectric layer adapted to contract upon the application of an electrical voltage and bend the first transparent layer, wherein the first transparent layer is applied on a substrate having a through going cavity being essentially symmetrical relative to an axis, and a transparent polymer positioned in said cavity and having a surface contact with said first transparent layer, wherein the first transparent layer is constituted by a hard material comprising glass, sapphire or SiO₂, and the piezoelectric layer is constituted by a ring shaped piezoelectric element positioned symmetrically relative to said axis such that a central area of said ring achieves an essentially spherical refractive surface upon the application of an electrical voltage.
 16. The optical element according to claim 15, wherein the first transparent layer is a constituted by SiO₂ and the substrate is made from Si.
 17. The optical element according to claim 15, wherein both the first transparent layer and the substrate are made from glass.
 18. The optical element according to claim 15, wherein the piezoelectric layer is a silk screen print deposited on said first transparent layer by screen printing.
 19. The optical element according to claim 15, further comprising a second transparent layer positioned on the opposite side of the polymer and essentially covering said polymer.
 20. The optical element according to claim 19, also including a leakage channel for equilibrating the pressure inside said element with the environment.
 21. The optical element according to claim 19, also including a predetermined amount of compressible gas inside said cavity.
 22. The optical element according to claim 21, wherein said compressible gas is distributed essentially symmetrically relative to said axis.
 23. The optical element according to claim 15, wherein the piezoelectric layer is adapted to provide a planar symmetric force relative to a plane including the optical axis, thus providing a cylindrically shaped bend on said first transparent layer.
 24. The optical element according claim 15 where the said polymer is a gel.
 25. A method for producing a lens comprising the steps of: providing a silicon plate having a SiO₂ or glass layer on one side; etching an opening into said silicon plate; positioning a soft polymer into said opening, the soft polymer thus having contact with the SiO₂ or glass layer; and positioning a ring shaped piezoelectric element on said SiO₂ or glass layer coaxially with a chosen optical axis, wherein said ring shaped piezoelectric element is adapted to provide a force to the SiO₂ or glass layer thus making a bulge or dome in said first layer and said soft polymer in a central area of said ring shaped piezoelectric element.
 26. The method according to claim 25, further comprising providing a second layer on an opposite side of said silicon plate so that the soft polymer is enclosed in a cavity.
 27. A method for providing a lens comprising the steps of: depositing a ring shaped piezoelectric actuator on one side of a glass substrate; thinning the glass substrate by etching or grinding from a side of said glass substrate opposite the one side to form a cavity with a chosen depth, with a thin transparent glass layer remaining; and placing a soft polymer at one of the sides of the thinned glass layer with the ring shaped piezoelectric actuator thus surrounding a central area positioned on the opposite side from said cavity. 